1. Field of the Invention
The present invention relates to a method of setting the output current of a monolithic integrated pad driver specifically designed for high data rates and high load capacitances.
2. Description of the Prior Art
A high current yield and high switching speed of such pad drivers are an important prerequisite for a high-speed exchange of ever increasing amounts of data between interacting data processing devices. Even within a large-scale integrated circuit, an efficient driver circuit may become necessary if a plurality of subcircuits, which represent an increased load capacitance, have to be connected to a digital signal source within the chip.
A known disadvantage of efficient pad drivers is that the value of the output current, henceforth also called "load current," is determined by the minimum current which still permits reliable data transfer under the worst possible technological and external conditions. As a result of the usual tolerances, the actual output current is thus generally much too large, e.g., by a factor of 5, and produces via the voltage-supply system voltage dips and capacitive or inductive interference signals which are difficult to control.
In addition, the transient current characteristic of prior art pad drivers is, as a rule, pulse-shaped with a distinct current maximum, so that the voltage dips in the supply system will be further increased. This mainly affects the analog signal-processing stages that may be present. During analog-to-digital or digital-to-analog conversion, interfering signal components are then produced which get into the useful-signal band through mixing effects.
According to the invention, such interference can be eliminated if the value and tolerance of the respective output current can be set and held more precisely by a suitable method and a circuit arrangement for carrying out the same.
Therefore, the method for setting the output current of pad drivers in accordance with the invention is not limited to the pad drivers of chip output terminals, but can also be applied to internal driver stages. The term "pad driver" therefore is to be understood in the broadest possible sense.
German Patent, DE-A-40 27 534, for example, discloses a CMOS pad driver in which the peak current is reduced by current-limiting means in order to eliminate interfering effects on the internal supply lines. The current limiting is provided by an additional circuit element in the manner of a junction diode in the shunt-current path of the complementary output transistors. The dimensions of the effective junction area of this diode determine the peak forward current.
It is the object of the invention as claimed to provide an improved method whereby the absolute value and the tolerance of the output current of pad drivers are more accurately settable and whereby the load-current peak, in particular, is prevented.